Semiconductor processing methods of forming transistors, semiconductor processing methods of forming dynamic random access memory circuitry, and related integrated circuitry

ABSTRACT

Semiconductor processing methods of forming transistors, semiconductor processing methods of forming dynamic random access memory circuitry, and related integrated circuitry are described. In one embodiment, active areas are formed over a substrate, with one of the active areas having a width of less than one micron, and with some of the active areas having different widths. A gate line is formed over the active areas to provide transistors having different threshold voltages. Preferably, the transistors are provided with different threshold voltages without using a separate channel implant for the transistors. In another embodiment, a plurality of shallow trench isolation regions are formed within a substrate and define a plurality of active areas having widths at least some of which being no greater than about one micron (or less), with some of the widths preferably being different. One or more gate lines may be coupled to the respective active areas to provide individual transistors, with the transistors corresponding to the active areas having the different widths having different threshold voltages. In another embodiment, two field effect transistors are fabricated having different threshold voltages without using a separate channel implant for one of the transistors versus the other.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a Divisional Application of U.S. patentapplication Ser. No. 09/834,660, filed Apr. 12, 2001, pending entitled“Semiconductor Processing Methods Of Forming Transistors, SemiconductorProcessing Methods Of Forming Dynamic Random Access Memory Circuitry,And Related Integrated Circuitry,” naming Luan C. Tran as inventor,which is a Divisional Application of U.S. patent application Ser. No.09/388,857, filed Sep. 1, 1999, the disclosure of which is incorporatedby reference.

TECHNICAL FIELD

This invention relates to semiconductor processing methods of formingtransistors, to semiconductor processing methods of forming dynamicrandom access memory circuitry, and to related integrated circuitry.

BACKGROUND OF THE INVENTION

Semiconductor processing typically involves a number of complicatedsteps which include patterning, etching, and doping or implanting steps,to name just a few, which are necessary to form desired integratedcircuitry. One emphasis on improving the methods through whichintegrated circuitry is formed, and which is directed to reducing theprocessing complexity, relates to reducing the number of processingsteps. By reducing the number of processing steps, risks associated withprocessing mistakes entering into the processing flow are reduced.Additionally, wherever possible, it is also highly desirable to reduceprocessing complexities while providing added flexibility in theprocessing itself.

For example, several processing steps are required to form transistorconstructions. One or more of these steps can include a thresholdvoltage definition step in which one or more channel implantation stepsare conducted to define the threshold voltage for the ultimately formedtransistor. In some applications, it is desirable to have transistorswith different threshold voltages. Typically, different thresholdvoltages are provided by additional masking and doping or implantingsteps to adjust the doping concentration within the channel region ofthe various transistors desired to have the different threshold voltage.Specifically, one transistor might be masked while another receives athreshold implant; and then other of the transistors might be maskedwhile the first-masked transistor receives a threshold implant.

This invention grew out of concerns associated with reducing theprocessing complexities involved in forming transistors having differentthreshold voltages.

SUMMARY OF THE INVENTION

Semiconductor processing methods of forming transistors, semiconductorprocessing methods of forming dynamic random access is memory circuitry,and related integrated circuitry are described. In one embodiment,active areas are formed over a substrate, with one of the active areashaving a width of less than one micron. A gate line is formed over theactive areas to provide transistors having different threshold voltages.Preferably, the transistors are provided with different thresholdvoltages without using a separate channel implant for the transistors.The transistor with the lower of the threshold voltages corresponds tothe active area having the width less than one micron.

In another embodiment, a plurality of shallow trench isolation (STI)regions are formed within a substrate and define a plurality of activeareas having widths at least some of which are no greater than about onemicron, with at least two of the widths preferably being different. Agate line is formed over the respective active areas to provideindividual transistors, with the transistors corresponding to the activeareas having the different widths having different threshold voltages.In an STI process, devices having width smaller than 1 micron typicallyalso have a lower threshold voltage. This is referred to as “reversednarrow width” effect as contrasted with the case of transistors formedusing LOCOS isolation, where threshold voltage tends to increase asdevice width decreases.

In another embodiment, two field effect transistors are fabricatedhaving different threshold voltages without using a separate channelimplant for one of the transistors versus the other.

In yet another embodiment, two series of field effect transistors areformed, with one series being isolated from adjacent devices by shallowtrench isolation, the other series having active area widths greaterthan one micron. The one series is formed to have active area widthsless than one micron to achieve lower threshold voltages than the otherof the series.

In yet another embodiment, one of the two series of field effecttransistors are isolated by shallow trench isolation, and differentthreshold voltages between the field effect transistors in differentseries are achieved by varying the active area widths of the fieldeffect transistors in the series. At least one of the series preferablyhas active area widths less than one micron.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic side sectional view of the semiconductor waferfragment in process in accordance with one embodiment of the invention.

FIG. 2 is a view of the FIG. 1 wafer fragment at a processing step whichis subsequent to that which is shown in FIG. 1.

FIG. 3 is a plan view of the FIG. 1 wafer fragment at a processing stepwhich is subsequent to that which is shown in FIG. 2.

FIG. 4 is a side view of the FIG. 3 wafer fragment.

FIG. 5 is a schematic diagram of circuitry formed in accordance withanother embodiment of the invention.

FIG. 6 is a schematic diagram of circuitry formed in accordance withanother embodiment of the invention.

FIG. 7 is a schematic diagram of circuitry formed in accordance withanother embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

Referring to FIG. 1, a semiconductor wafer fragment in process is showngenerally at 10, and includes a semiconductive substrate 12. In thecontext of this document, the term “semiconductive substrate” is definedto mean any construction comprising semiconductive material, including,but not limited to, bulk semiconductive materials such as asemiconductive wafer (either alone or in assemblies comprising othermaterials thereon), and semiconductive material layers (either alone orin assemblies comprising other materials). The term “substrate” refersto any supporting structure, including, but not limited to, thesemiconductive substrates described above.

Referring to FIG. 2, a plurality of active areas are formed oversubstrate 12, with an exemplary pair of active areas 14, 16 being shown.Active areas 14, 16 can constitute individual active sub-areas within alarger active area. In a preferred embodiment, active areas or sub-areas14, 16 are defined between a plurality of shallow trench isolationregions 18 which are received within substrate 12. The spacing ofshallow trench isolation regions 18 defines a plurality of active areawidths, with exemplary widths being shown at w₁ and w₂. Preferably, atleast two of the widths are different from one another. Of course, morethan two of the widths could be different from one another.

In one embodiment, some of the active area widths are no greater thanabout one micron. One micron happens to be a break point that istechnologically dependent. In other words, STI transistors show athreshold voltage reduction with reducing gate width when the gate widthis about one micron or less. It will be understood that other sizes thatcorrespond to a break point in threshold voltage versus gate width orcontrol element size for transistors made using other technologies couldbe used instead of “one micron”.

In one embodiment, one or both of widths w₁ and w₂ could be less thanone micron. In a preferred embodiment, the different active area widthsimpart to transistors which are to be formed, different thresholdvoltages which, in a most preferred embodiment, are achieved withoutconducting or using a separate channel implant for the differenttransistors. Such results in a reduction in the number of processingsteps which were previously required to form transistors havingdifferent threshold voltages.

In one embodiment, the different threshold voltages are each less thantwo volts. In another embodiment, the different threshold voltages areeach less than one volt. In this example, the transistor having thelower of the threshold voltages corresponds to the transistor which isformed relative to the active area having the lesser or smaller activearea width.

With respect to provision of the channel implant(s) which defines thethreshold voltages, one or more such implants can be conducted relativeto the active areas. Preferably, each of the one or more channelimplants are common to the transistors having the different active areawidths which, in turn, provides transistors having different thresholdvoltages.

FIG. 3 is a plan view of the FIG. 1 wafer fragment at a processing stepwhich is subsequent to that which is shown in FIG. 2, and FIG. 4 is aside view of the FIG. 3 wafer fragment. A transistor gate line 20 isformed over respective active areas 14, 16 to provide individualtransistors, wherein the transistors corresponding to the active areashaving the different active area widths have different thresholdvoltages as discussed above. Gate lines such as line 20 typically have agate oxide layer, one or more conductive layers such as polysilicon anda silicide layer, one or more insulative caps, and insulative sidewallspacers (not shown), none of which are specifically designated. Theillustrated gate line constitutes a common gate line which is formedover the illustrated active areas. It is, of course, possible to formseparate gate lines over the active areas having the different widths.

Alternately considered, and in accordance with one embodiment of thepresent invention, two series of field effect transistors are formedover substrate 12. One of the series of field effect transistors (anexemplary transistor of which being formed over active area 14) isisolated from other adjacent devices by shallow trench isolation regions18. The other series of field effect transistors (an exemplarytransistor of which being formed over active area 16) has active areawidths greater than one micron, with the first-mentioned series beingformed to have active area widths less than one micron to achieve lowerthreshold voltages than the other of the series. Preferably, thethreshold voltages for the two series of field effect transistors aredefined by one or more common channel implants. In a most preferredembodiment, the one or more common channel implants are the onlyimplants which define the threshold voltages for the two series of fieldeffect transistors.

Further and alternately considered, and in accordance with anotherembodiment of the present invention, the two series of field effecttransistors just mentioned include at least one series which is isolatedfrom adjacent devices by shallow trench isolation regions such asregions 18. Different threshold voltages are achieved between fieldeffect transistors in the different series by varying the active areawidths of the field effect transistors in the series, with at least oneof the series having active area widths less than one micron, or lessfor future technologies.

Accordingly, field effect transistors can be fabricated having differentthreshold voltages without using a separate channel implant for thefield effect transistors having the different threshold voltages. Suchcan result in a reduction in processing steps, which formerly includedadditional masking steps. One or more of the active areas can havewidths less than one micron, with such widths being varied in order tochange the threshold voltages of the transistors formed thereover.

In operation, various methods of the invention provide integratedcircuitry having transistors with different threshold voltages withoutthe added processing complexity. In a preferred embodiment, variousmethods of the invention can provide dynamic random access memorycircuitry having a memory array area for supporting memory circuitry anda peripheral area for supporting peripheral circuitry. A plurality ofshallow trench isolation regions are received within the peripheral areaof the substrate and define a plurality of active areas having widthswithin the substrate, some of the widths being no greater than about onemicron. Preferably, at least two of the widths are different. Aconductive line is formed or disposed over the respective active areasto provide MOS gate electrodes for individual transistors. Thetransistors corresponding to the active areas having the differentwidths preferably have different threshold voltages. Exemplary dynamicrandom access memory circuitry is described in U.S. Pat. Nos. 5,702,990and 5,686,747, which are incorporated by reference.

Referring to FIG. 5, a circuit 28 is provided and includes transistors30, 32. Such transistors can be fabricated, in accordance with themethods described above, to have different threshold voltages. In thisexample, transistor 30 serves as a pass transistor and has a lowthreshold voltage V_(t1), while transistor 32 serves as a switchingtransistor and has a high threshold voltage V_(th).

Referring to FIG. 6, a circuit 34 is provided and includes transistors36, 38 which can have different threshold voltages. Such circuitcomprises a portion of precharge circuitry for dynamic random accessmemory circuitry. In the example of FIG. 6, the transistor 36 has a lowthreshold voltage V_(t1) and the transistor 38 has a high thresholdvoltage V_(th).

Referring to FIG. 7, a circuit is shown generally at 40 and comprisestransistors 42, 44 and 46 having threshold voltages V_(t1), V_(t2) andV_(t3), respectively. The transistors 42, 44 and 46 are fabricated to beformed in a parallel configuration with a common gate line 48interconnecting the transistors 42, 44 and 46 and coupling a signalC_(SAL) to gates of the transistors. In this example, the transistors42, 44, 46 each have different active area widths which results indifferent threshold voltages.

Also shown in FIG. 7 is a sense amplifier circuit 50 includingcross-coupled transistors 52 and 54. In one embodiment, the transistors52 and 54 are formed to have a low threshold voltage V_(t1). When thesignal CSAL goes to logic “1”, the common node labeled RNL* equilibratesthe potentials on sources of the transistors 52 and 54 in preparationfor reading stored data from memory cells in a memory array (not shown).In the example shown in FIG. 7, the circuit 40 acts as a pull-downcircuit and equilibrates the node RNL* to ground. Use of multipletransistors 42, 44 and 46 having different threshold voltagesfacilitates (“softens”) sensing at the beginning of the sensing cycleand also facilitates more rapid sensing at the end of the cycle whendifferential signals have been developed by the transistors 52 and 54.

Advantages of the invention can include provision of a plurality oftransistors having different threshold voltages, without the necessityof providing different dedicated processing steps to achieve suchdifferent threshold voltages. In various preferred embodiments, suchresults are attained through the use of shallow trench isolation andvarious so-called reverse narrow width characteristics. Additionally,current drive can be achieved using multiple narrow width devices inparallel of the same or different V_(t)'s. The invention ca be usefulfor low threshold voltage applications such as precharge circuitry inDRAM circuitry, or as output drivers where low threshold voltages areimportant to obtain higher signal levels.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A transistor assembly comprising: a plurality of active areas havingwidths defined by shallow trench isolation regions of no greater thanabout one micron, at least some of the widths being different; and gatelines disposed over the plurality of active areas to provide individualtransistors, those transistors whose widths are different havingdifferent threshold voltages from one another.
 2. The transistorassembly of claim 1, wherein the threshold voltages of at least some ofthe individual transistors are less than one volt.
 3. The transistorassembly of claim 1, wherein individual transistors having active areaswith the smaller widths have threshold voltages which are smaller thanother individual transistors having active areas with larger widths. 4.The transistor assembly of claim 1, wherein one of the individualtransistors comprises a portion of precharge circuitry for dynamicrandom access memory circuitry.
 5. The transistor assembly of claim 1,wherein one of the individual transistors comprises a pass transistor.6. The transistor assembly of claim 1, wherein one of the individualtransistors comprises a portion of sense amplifier circuitry for dynamicrandom access memory circuitry and has a lower threshold voltage V_(t1).7. The transistor assembly of claim 1, wherein some of the individualtransistors are joined together in a parallel configuration.
 8. Thetransistor assembly of claim 1, wherein the gate lines are disposed in adirection over the plurality of the active areas, and wherein the widthsof the active areas are defined along the direction.
 9. The transistorassembly of claim 1, wherein the widths of the plurality of the activeareas are less than one micron.
 10. The transistor assembly of claim 1,wherein the individual transistors comprise an electrically parallelcircuit configuration.
 11. The transistor assembly of claim 1, whereinthe gate lines comprise a separate and distinct gate line for each ofthe plurality of the active areas.